Mention geophysics and one could be excused for thinking of Time Team enthusiasts tracing Neolithic dwellings, or perhaps seismic images of petroleum reserves deep below the seabed. For a growing number of rail engineers, however, geophysics is increasingly becoming a tool of the trade, particularly in the area of track maintenance.

A whole suite of geophysical methods are being used throughout the railway infrastructure lifecycle to provide information on condition and construction at depths ranging from a few centimetres to hundreds of metres.

For the near-surface, ground penetrating radar (GPR) is the most widely recognised geophysical technique for trackbed assessment, with surveys undertaken at both network and project level. Variants of the same technique are used to map buried services and other shallow features.

For an understanding of subsurface structure and condition at greater depth, there are many more non-destructive techniques for deriving physical data based around measurement of electrical, magnetic, acoustic and gravitational energy. With powerful ground profiling capabilities, they can be integrated with more traditional intrusive investigations (coring, drilling) to reduce uncertainty relating to ground structure and material properties.

Engineering geophysics specialists at Fugro have been applying their capabilities in subsurface investigation to the specific challenges of the railway sector for well over a decade, both in the UK and further afield.

Asset management

A short hop across the Irish Sea provides a topical example of the benefits of GPR for enhancing the knowledge base crucial to efficient railway asset management.

In a major project for Irish Rail, Fugro surveyed more than 1,000 km of railway trackbed using a train- mounted ground penetrating radar system integrated with the client’s survey vehicle to simplify logistics.

With antennae mounted at the front and rear of the Irish Rail Track Recording Vehicle, the team collected six continuous data-streams at a normal operational speed of 65 km/hr. Coverage included the mainline passenger routes between Cork, Galway, Sligo and Westport.

Fugro’s transport team resolved challenges of equipment set-up and the simultaneous collection of multiple data- streams to complete the survey in just seven days without disruption to scheduled services.

Irish Rail plans to use the data to help determine the condition and thickness of track ballast, as well as ballast formation, sub-formation and presence of water. This will assist engineers in assessing, prioritising and designing track rehabilitation work and associated drainage improvements.

Irish Rail project engineer, Sarah Ross, said: “The GPR data will be used to assess trackbed conditions on the network including indications of ballast fouling, poor formation and underlying issues affecting track geometry. It will help us formulate more effective maintenance solutions and a better understanding of the underlying problems in areas prone to poor track geometry with a view to improving overall track quality and ride comfort for passengers.”

The resulting data is supplied in a range of formats compatible with the client’s GIS system and for analysis using track management software.

Investigative expertise

The success of the project has drawn on Fugro’s comprehensive expertise in trackbed investigation. Project manager, Charles Baker, said: “A key objective was to establish a rigorous system of radar collection/interpretation for ranking and comparing trackbed condition across the whole rail network. The data will allow the client to identify locations requiring further investigation and intervention, as well as develop a long term maintenance plan.”

Irish Rail is among a number of progressive rail operators using Fugro’s radar as a long term asset management tool. Investment in this type of rail network survey can help operators target and plan maintenance more effectively, delivering considerable long-term cost savings.

GPR delivers not only accurate, reliable and continuous data, it is also non-intrusive and has a relatively light footprint in terms of the people and plant needed on-track. Add to this the speed of coverage, then radar is extremely cost- and time-efficient for comprehensive data-gathering, city to city or network wide.

GPR and other geophysical surveys provide a more robust body of knowledge than reliance on desk studies and record searches alone. They are also far more cost- effective to mobilise compared with the heavyweight plant required for ‘conventional’ intrusive ground investigations.

Complex structures

The subsurface of the UK is a complex place. Hidden relics of past industrial activity are intertwined with naturally occurring geohazards, posing a threat to the safe and efficient operation, maintenance and upgrade of the rail network.

These have often been the target of interest in the many rail surveys undertaken by Fugro. Investigations of man-made cavities and structures have included tin mines in Cornwall, ‘lost’ culverts in Somerset and hidden tunnel construction shafts throughout the UK. The company has even been asked to map the extent of an underground fire in a rail embankment in north-east England. The list of natural geohazards tackled is similarly varied, ranging from swallow holes and other karst features to coastal tracks being undercut by the sea and inland embankments being undermined by badgers.

As well as determining ground stratification and finding faulting, cavities and objects that present an engineering risk, geophysical methods can provide the engineer with the data required to construct a reliable ground model.

Much can be gained from analysis of the strength and velocity of seismic (acoustic) energy transmitted through the ground. This can be
achieved using an impressive combination of surface and downhole acoustic sources, ranging from a hearty blow to the ground with a 10 kg hammer to sophisticated vibroseis trucks that generate highly controlled ground vibrations.

By analysing different elements of the energy reflected from material boundaries, geophysicists can hand the engineer a report including a suite of data relating to the elastic properties of soil and rock including stiffness, rippability, and specific measures such as shear, bulk and Young’s Modulii.

Flexible techniques

Whether profiling long tranches of trackbed, assessing embankments, or evaluating ground strength for new construction, modern geophysical techniques are flexible, fast and cost-effective to apply in the challenging and often sensitive railway environment.

With more pressure on the rail system to increase output at lower cost and risk, a geophysical perspective could be key to the leaner, better-informed decision-making that will be required in railway engineering in the decade ahead.

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